This invention relates to a method of recovering alkali values from trona ore. In particular, this invention relates to a sequential crystallization process to recover separately sodium bicarbonate and sodium carbonate from dissolved trona.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O). A vast deposit of mineral trona is found in southwestern Wyoming near Green River. This deposit includes beds of trona and mixed trona and halite (rock salt or NaCl) which covers approximately 2,600 km.sup.2. The major trona beds range in size from less than 428 km.sup.2 to at least 1,870 km.sup.2. By conservative estimates, these major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale. The quality of the trona varies depending on its particular location in the stratum.
A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1 ______________________________________ Constituent Weight Percent ______________________________________ Na.sub.2 CO.sub.3 43.6 NaHCO.sub.3 34.5 H.sub.2 O (crystalline and free 15.4 moisture) NaCl 0.01 Na.sub.2 SO.sub.4 0.01 Fe.sub.2 O.sub.3 0.14 Insolubles 6.3 ______________________________________
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water and dissolves to yield approximately 5 parts by weight sodium carbonate (Na.sub.2 CO.sub.3) and 4 parts sodium bicarbonate (NaHCO.sub.3), as shown in the above analysis. The trona ore is processed to remove the insoluble material, the organic matter and other impurities to recover the valuable alkali contained in the trona.
One such valuable alkali produced from trona is soda ash (a commercial grade of sodium carbonate). Soda ash is one of the largest volume alkali commodities made in the United States. In 1992, trona-based soda ash from Wyoming comprised about 90% of the total U.S. soda ash production. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
A common method to produce soda ash from trona ore is known as the "monohydrate process," which consumes great quantities of water (a scarce and valuable resource in Wyoming) and energy. In that process, crushed trona ore is calcined (i.e., heated) at a temperature between 125.degree. C. and 250.degree. C. to convert sodium bicarbonate into sodium carbonate and form crude soda ash.
During calcination, the sodium sesquicarbonate in the trona ore breaks down into sodium carbonate, carbon dioxide and water. Also, calcination releases some of the organics associated with trona or trona shale.
The resulting crude sodium carbonate and the released organics are then dissolved in water. After dissolving the calcined trona, any undissolved solids are then removed and the solution is treated with activated carbon to remove some of the dissolved organics. The solution is then filtered. One of the perceived advantages of the monohydrate process is that calcined trona dissolves faster than raw trona. Another perceived advantage is that dissolved calcined trona produces a more concentrated sodium carbonate solution of about 30%, while dissolved raw trona produces a solution having only about 16% sodium carbonate plus 10% sodium bicarbonate.
The filtered solution of sodium carbonate is fed to an evaporative crystallizer where some of the water is evaporated and some of the sodium carbonate forms into sodium carbonate monohydrate crystals (Na.sub.2 CO.sub.3.H.sub.2 O). A slurry containing these monohydrate crystals and a mother liquor is removed from the evaporators and the crystals are separated from the mother liquor. The crystals are then calcined, or dried, to convert it to dense soda ash. The mother liquor is recycled back to the evaporator circuit for further processing into sodium carbonate monohydrate crystals.
Presently, the monohydrate process consumes considerable amounts of water. When the trona is calcined, the natural water content is evaporated (as shown in Table 1, water makes up about 15% of the trona ore). Yet after that water is evaporated off, more water must be added to dissolve the calcined trona.
In addition to consuming water, the monohydrate process consumes and wastes considerable amounts of energy. Calcining equipment, for example, has only about 50% energy efficiency. Much energy is also wasted in calcining simply to raise the temperature of the sodium carbonate in the trona up to the calcining temperature even though the sodium carbonate itself does not undergo conversion. Finally, significant energy is wasted during calcining in evaporating naturally occurring water in the trona.
Another disadvantage of the monohydrate process is that the feed solution can only contain a maximum of about 30% sodium carbonate. The evaporative crystallizers could be more efficiently operated if a more concentrated feed solution were used.
Therefore, there is a need to provide a simple, energy-efficient and water-conserving process to recover alkali values from trona ore.